This invention pertains generally to lasers and particularly to resonant cavities in such devices.
It is known in the art that the end walls of a resonant cavity in a laser may be defined by parallel plane mirrors, one of which is totally reflective and the other of which is partially reflective. When laser action occurs, constructive interference is experienced only by light having a desired wavelength. As a result then, the radiant energy from the laser is formed into a highly collimated solid beam passing through the partially reflective mirror. Unfortunately, such beam is concentrated on the partially reflective mirror with the result that, even though only a relatively small portion of such beam may be absorbed by the mirror material, sufficient localized heating may be experienced to induce appreciable thermal strains in the partially reflective mirror. As a matter of fact, such strains, which increase with an increase in flux density of the laser energy, ultimately become great enough to cause spontaneous fracturing of the partially reflective mirror.
It is known in the art that the flux density of laser energy on the partially reflecting mirror in a laser cavity may be reduced by directing the laser energy toward a ring-shaped area on such mirror. Thus, by arranging, as shown in U.S. Pat. No. 3,792,370, opposing mirrors within a laser cavity, the laser energy may be spread out to cover a ring having a much larger area than the area covered by the laser energy in a cavity using only parallel mirrors. Consequently, for equal amounts of laser energy, the arrangement shown in U.S. Pat. No. 3,792,370 is less susceptible to damage from thermal strains within the partially reflective mirror. Although an arrangement as just mentioned makes it possible to increase the amount of laser energy passing through the partially reflective mirror in an optical cavity, it is desirable to permit even greater amounts of laser energy to be passed through such a mirror.
It is also known in the art that either the partially reflective mirror or the totally reflective mirror (or both such mirrors) making up the end walls of an optical cavity of a laser may correspond to zones of selected spheres. According to the art, however, the spacing between such end walls is dependent upon the radius of curvature of the curved mirror (if one is curved) or upon the radii of curvature of the two mirrors (if both are curved). Such dependence, in turn, makes it impossible to change the length of the optical cavity without changing the curvature of either, or both, end walls.
Therefore, it is a primary object of this invention to provide an improved laser wherein the density of laser energy passing through a partially reflective mirror in an optical cavity in such laser may be reduced.
Another object of this invention is to provide an improved laser as just mentioned, the optical cavity within such laser being adapted to support laser energy in a hollow cylindrical beam-like configuration.
Still another object of this invention is to provide an improved laser as mentioned above, such laser utilizing at least a curved partially reflective mirror to define one end wall of an optical cavity, the length of such cavity being independent of the curvature of such mirror.